The present invention relates to an ion beam milling apparatus and method including a specimen holder for the preparation of transmission electron microscope specimens, and more particularly to a specimen holder and method which permits two-sided milling at very low angles down to 0.degree..
Transmission electron microscopy is an important technique for studying the detailed microstructure of many materials. Improvements in the design and operation of electron microscopes have generated considerable interest in obtaining atomic resolution images of various materials. However, the preparation of specimens for atomic resolution transmission electron microscopy is demanding, requiring a final specimen which is very thin (i.e., &lt;50 nm) and free from artifacts. Typically, specimen preparation involves initial slicing, sectioning, trepanning, and/or grinding operations to produce a relatively thin (100-200 .mu.m) disk of approximately 3 mm diameter.
Ion beam milling systems have been used to prepare specimens of various materials including ceramics, semiconductors, metals, and combinations thereof for atomic resolution transmission electron microscopy. In such ion beam milling systems, such as the system disclosed in commonly-assigned U.S. Pat. No. 5,009,743, to Swann, specimens are mounted on holders and placed in the path of one or more ion beams. The ion beams gradually remove atoms from the surface of the specimen until a small perforation is formed in the center of the specimen. Generally, the area around the perforation is then thin enough (i.e., &lt;50 nm) for atomic resolution analysis by a transmission electron microscope.
However, ion milling is a slow process. To increase the rate at which atoms are removed from the surface of the specimen, it is a common practice to mill the specimen at higher angles of from 15.degree.-25.degree. between the incident beam and the specimen surface. Such higher angles maximize the effects of the beam but may cause artifacts such as crystallographic imperfections and amorphous surface layers which result from the penetration of energetic ions and atoms into the surface of the specimen.
The total milling rate may also be increased through the use of two or more ion guns simultaneously from both sides of the specimen. Prior art specimen holders have been designed to accommodate high angle milling, milling from both sides, and specimen rotation to obtain more uniform thinning across the specimen surface. Such holders have included a pair of annular thin metallic plates, called clamping plates, having openings of a diameter just less than the diameter of a typical specimen (i.e., about 3 mm). The specimen is placed between the clamping plates concentrically within the annular openings. The entire assembly is then clamped together and mounted in an auxiliary holder to provide rotation.
Since ion beams are collimated and not focused, they are relatively broad in comparison to specimen size. Thus, in addition to striking the specimen, the ion beams strike the clamping plates as well as other parts of the auxiliary holder. This can result in the deposition of sputtered material from the clamping plates and auxiliary holder onto the specimen creating troublesome artifacts on the surface(s) of the specimen. The thinning of the clamping plates and auxiliary holder due to this sputtering necessitates frequent replacement.
Another problem with conventional clamping plates is that their thickness limits the minimum attainable milling angle for the specimen. This is because the clamping plates must have some mechanical strength, which requires that they have a certain minimum thickness. Clamping plates with somewhat greater thicknesses than that which is absolutely necessary are sometimes favored because they have a longer useful life, especially in view of the thinning which results from repeated use.
While higher milling angles produce more rapid milling of the specimen surface, ion milling at glancing angles of about 5.degree. or less has become increasingly popular because it produces higher quality specimens and fewer artifacts, especially specimens composed of materials which are difficult to mill. For example, in some two-phase specimens, one phase is milled more rapidly than the other. Specimens milled at higher angles are of lower quality and include surface roughness, limited thin areas, and differential thinning rates for different constituents in the specimen; low angle milling overcomes many of these problems.
A major requirement for low angle ion milling is a specimen holder with an appropriate line-of-sight capability. That is, the specimen holder must permit one or more ion beams to be directed against the specimen at angles down to 0.degree.. An example of a specimen holder having low angle milling capability is the holder disclosed in the above-mentioned U.S. Pat. No. 5,009,743. In that patent, specimens are seated on the top face of a pedestal, permitting one or more ion beams to be directed at one side of the specimen at milling angles down to 0.degree.. The holder includes a raised lip on the upper surface of the pedestal to secure the specimen. However, such a holder does not permit two-sided milling or removal of any contaminants from the underside of the specimen. Further, the raised lip is milled away by the action of the ion beams, necessitating frequent replacement of the specimen holder.
Accordingly, the need still exists in this art for ion milling apparatuses including specimen holders which permit simultaneous two-sided ion beam milling at very low angles down to 0.degree..